1. Field of the Technology
The present technology relates to an optical pickup apparatus that emits light to an optical recording medium and receives light reflected from the optical recording medium for performing recording or reproduction of information on the optical recording medium.
2. Description of the Related Art
As a related art, an optical pickup apparatus is described in Japanese Unexamined Patent Publication JP-A 2007-287278. The optical pickup apparatus, being designed to perform recording and reproduction of information on two different types of optical recording media, utilizes light of two different wavelengths, namely light of a first wavelength and light of a second wavelength. An example of the optical pickup apparatus based on this related art is shown in FIGS. 11, 12, 13A, 13B, and 14A to 14C.
FIG. 11 is a view showing one example of the structure of an optical pickup apparatus 120 of the related art. FIG. 12 is a plan view showing a diffractive element 121 and a light-receiving element 122 of the optical pickup apparatus 120. FIGS. 13A and 13B are plan views showing the diffractive element 121 and the light-receiving element 122 subjected to light.
FIG. 13A shows a state of receiving light of the first wavelength, and FIG. 13B shows a state of receiving light of the second wavelength. In the case of using the first wavelength, a spot 136 of light reflected from an optical recording medium 11 is displaced rightward from a spot 135 of light incident on the optical recording medium 11 from a light emission point 123. In the case of using the second wavelength, a spot 137 of light reflected from the optical recording medium 11 is displaced leftward from the light spot 135 incident on the optical recording medium 11 from the light emission point 123.
An emission wavelength from the light emission point 123 for the first-wavelength light is shorter than an emission wavelength from a light emission point 124 for the second-wavelength light. The angle of diffraction of a light beam 131 from the light emission point 123 for the first-wavelength light in the diffractive element 121 is smaller than the angle of diffraction of a light beam 132 from the light emission point 124 for the second-wavelength light in the diffractive element 121. With consideration given to the difference in diffraction angle, the light-receiving element 122 has light-receiving regions 18a to 18l arranged in three rows 125a to 125c. 
The light beam 131 from the light emission point 123 for the first-wavelength light is incident on, among the light-receiving regions arranged in the three rows 125a to 125c, the light-receiving regions 18i and 18k belonging to the row 125a corresponding to a smaller diffraction angle, and also the light-receiving regions 18a to 18h belonging to the central row 125b. When electric signals outputted from the light-receiving regions 18a to 18l, respectively, are defined as Sa to Sl, respectively, a focus error signal (hereafter referred to as “FES”) can be calculated in accordance with a formula of FES=(Sa+Sc)−(Sb+Sd).
A tracking error signal (hereafter referred to as “TES”) can be detected by the three-beam method, the differential push pull method (hereafter referred to as “DPP method”), or the differential phase detection method (hereafter referred to as “DPD method”).
When a tracking error signal obtained by the three-beam method, a tracking error signal obtained by the DPP method, and a tracking error signal obtained by the DPD method are represented as TES1, TES2, and TES3, respectively, these tracking error signals can be calculated in accordance with formulae of TES1=(Se+Sg)−(Sf+Sh); TES2=(Sa+Sb)−(Sc+Sd)−k((Se+Sf)−(Sg+Sh)); and TES3=ph((Sa+Sb)−(Sc+Sd)), respectively. In the equations, k represents a constant for correcting the difference in intensity between a main beam and a sub beam, and ph( ) represents computation of a phase component of each signal inside the parentheses. The main beam is zero-order diffraction light produced by a light-dividing element 130, whereas the sub beam is ±first-order diffraction light produced by the light-dividing element 130.
FIGS. 14A to 14C are views showing light spots of a main beam and two sub beams on the diffractive element 121. More specifically, in FIGS. 14A to 14C, there are shown light spots of a main beam and two sub beams reflected from the optical recording medium 11 in the case of adopting the three-beam method for use with the second-wavelength light. FIG. 14A shows a light spot 126 of one of the sub beams; FIG. 14B shows a light spot 128 of the main beam; and FIG. 14C shows a light spot 127 of the other one of the sub beams. A dividing line in each of the light spots 126, 127, and 128 corresponds to the projected dividing line of the diffractive element 121 shown in FIG. 13B. Sub beams which pass through, among individual regions obtained by dividing the entire area by the dividing lines of the diffractive element 121 shown in FIG. 13B, regions 19e, 19g, 19f, and 19h, respectively, are incident on the light-receiving regions 18e, 18g, 18f, and 18h, respectively. On the other hand, sub beams which pass through regions 19m, 19n, 19o, and 19p, respectively, are not incident on the light-receiving regions but incident on other regions than the light-receiving regions.
According to the related art, in the case of adopting the three-beam method for use with the second-wavelength light, among the three beams incident on the diffractive element 121, the light spots 126 and 127 of sub beams are displaced toward a plus side and a minus side, respectively, in a tangential direction Y. Moreover, a boundary between a region 17a and a region 17b, as well as a boundary between a region 17c and a region 17d, of the diffractive element 121 is displaced in a radial direction X. Such a displacement takes place due to a slight sub-beam rotation required to cause a sub beam spot which is incident on the optical recording medium 11 to enter a location displaced from a main beam spot by a distance equivalent to one-half of track width. When the dividing line of the diffractive element 121 is projected on the light spot of each beam, as shown in FIGS. 14A to 14C, the dividing line of the diffractive element 121 corresponding to the contour of the light spot 126, 127 of sub beam is displaced in the radial direction X and the tangential direction Y as well with respect to the dividing line of the diffractive element 121 corresponding to the contour of the light spot 128 of main beam.
Accordingly, under the three-beam method, the tracking error signal TES1 can be calculated in accordance with a formula of TES1=(Se+Sg)−(Sf+Sh) on the basis of the electric signals Se, Sf, Sg, and Sh derived from the light-receiving regions 18e, 18f, 18g, and 18h for receiving the sub beam of ±first-order diffraction light. However, due to the absence of light-receiving regions for receiving light beams passing through the regions 19m, 19n, 19o, and 19p, respectively, of the diffractive element 121, the light beams passing through the regions 19m, 19n, 19o, and 19p are not detected as electric signals. Furthermore, since the sum total of the areas of the regions 19e and 19g for receiving the light spot 126 of the sub beam of +(positive) first-order diffraction light differs from the sum total of the areas of the regions 19f and 19h for receiving the light spot 127 of the sub beam of − (negative) first-order diffraction light, it follows that variation in electric signal ratio occurs between the electric signal Se+Sg and the electric signal Sf+Sh. After all, the tracking error signal TES1 is brought into an unbalanced state; that is, deviates from the value at which equality in electric signal ratio is attained. This leads to the instability of tracking servo.